Not already invented???  The images that come to mind are the old Rube Goldberg contractions where every piece of the creation was based on some simple machine: lever, winch, inclined plane, screw, etc., but the combination of all of them did something never done before.  An ingenious doorbell, something to get you out of bed in the morning (like a level Mom can pull to shake Junior's bed), a rain detector (rain fills cup, tips a lever on a fulcrum, ...), a better mousetrap, a device to scare whatever animal is eating your flower garden up.  Does any of that trigger something?

Look through the artwork gallery on rubegoldberg.com (link below) to spark some creative thinking.